1. Field of the Invention
The present invention pertains in general to a method of controlling the CVT in a vehicle powertrain. More particularly, the invention relates to a method of control for a kinetic hybrid vehicle, where the prime mover is either a motor, an engine, or a combination of both, and a flywheel acts as both a secondary power source and a secondary energy storage.
2. Description of the Related Art
Transportation accounts for a significant portion of global energy use and pollution, and there have been considerable efforts worldwide to promote new technology in transportation to improve fuel efficiency, aimed at promoting energy independence and reducing greenhouse gas emissions. As a result a market for hybrid vehicles and for electric vehicles has emerged, and many of the technologies being developed with raising the corporate-average fuel economy in mind have been directed at electric and hybrid vehicles.
Electric vehicles (EVs) and electric hybrids are able to bring improvements in efficiency from two main principles. One is to operate the primary power source (be it a motor or an engine) on the ideal operation line (IOL) for optimal efficiency. The other is to try to recover the vehicle's kinetic energy during deceleration (regenerative braking), which would otherwise be dissipated as heat in the brakes.
In conventional vehicles, efficiency is generally inversely related to performance. Engines should typically operate at a relatively high load for optimal efficiency, so relatively small engines are more efficient. However, relatively small engines have less reserve power, leading to poorer acceleration performance.
There are two components to the energy used to accelerate a vehicle: a portion used to overcome resistive forces like rolling resistance and air drag, and a portion used to overcome the vehicle's own inertia. Typically a vehicle requires its power source to provide a relatively high torque during acceleration, primarily to overcome inertia. When the vehicle is not accelerating or climbing a hill, the operation state of its primary power source lies below the IOL, resulting in low efficiency during most of the drive.
In a kinetic hybrid, where the vehicle's kinetic energy is recovered, stored, and reused kinetically, such as with a flywheel, countering the undesired effect of the vehicle's inertia is possible. Since a flywheel is a passive device, it should generally be controlled with a continuously variable transmission, or CVT, and control strategy is crucial in deciding how effectively the flywheel can contribute to increasing the vehicle's efficiency and performance. If the control is ineffective, the passive flywheel can become a “dead weight”.